This invention relates to oscillator circuits. More specifically, this invention relates to adjustable frequency oscillator circuits having an adjustable element for conveniently setting the frequency of the oscillator. One aspect of the invention is an adjustable frequency oscillator circuit, the output frequency of which is relatively immune to variations in the characteristics of the frequency setting element.
This invention also relates to an adjustable time delay circuit which may include a voltage controlled oscillator circuit for actuating a counter chain which counts a pre-selected number of oscillator cycles and produces an output change of state after the counter chain has filled up. More particularly, the invention relates to a time delay circuit for use in a time delay relay.
Time delay relays have many uses. All such uses involve the same type of function, i.e., a controlled operation occurs a predetermined period of time after a controlling operation
Adjustable time delay circuits are well-known in the art. Those circuits use a potentiometer for adjusting the delay period. In most classic adjustable timer circuits, the potentiometer is a resistance element in an "RC" circuit. The time delay circuit may be an oscillator based circuit, such as a classic CMOS oscillator, or a single charge event circuit, such as a unijunction transistor (UJT) based or programmable unijunction transistor (PUT) based circuit A PUT time delay relay, for example, is described in U.S. Pat. No. 3,689,807. These known circuits generally use the potentiometer as a two-terminal connected variable resistor, or possibly connect the wiper to one end of the potentiometer either directly or through another fixed resistor. Connecting a resistor from the wiper to one end of the potentiometer has the additional and usually undesirable effect of introducing a non-linearity into the potentiometer "rotation versus resistance" relationship. In any case, the effect is the same: the potentiometer remains the "R" in an "RC" based time delay circuit, and therefore any change in the potentiometer resistance value produces a corresponding change in the time constant or oscillator period.
In a classic CMOS oscillator, a potentiometer is used for adjusting the oscillator frequency. However, the low cost potentiometers usable for mass production of time delay relays have significant impedance tolerances. A typical potentiometer has a tolerance of plus or minus 20-30% "out of the box", and then any environmentally induced changes are in addition to that. If the potentiometer, as mentioned above, is used as a two-terminal connected variable resistor in a timing circuit for adjusting the oscillator period, any drift or tolerance shift in the potentiometer impedance value has a directly proportional effect on the oscillator period, and in order to be sure that each finished assembly is within its advertised tolerance band (for maximum time adjustment specifications), each potentiometer has to be matched within appropriate capacitor value, or electrically paralleled with a resistor of an appropriate size. This situation mandates grading and selection procedures in order to match the appropriate components for a given oscillator period, which would result in a given "time out" period.
While those grading and selection processes are undesirable in all methods of manufacturing time delay relays, these steps are totally impractical when using surface mounted components for the assemblies because those components are normally handled by automated equipment, with which component selection processes are either not possible or at least not very practical.